Cathode return bias circuit



Jan. 7, 1941. J. 1 FINCH 2,227,493

' ormoni: RETURN Bus clncur'r Filed Nov. 9. 1957 2 sheets-sheet 1 HIP -ilH- Jan. 7, 1941. Jgl.. FlNcH CATHODE RETURN BIAS CIRCUIT 2 Sheets-Sheet 2 Filed Nov. 9, 1937 .05 ,4MPERE5 PER D/SC INVENTOR. J @MES ./.1 F/NCH ATTORNEY.

' atented Jan 7 1941 2,227,493 cA'IHopE RETURN BIAS CIRCUIT James Leslie Finch, Patehogue, N. Y., assigner to Radio Corporation of Delaware America., a. corporation of Application November 9, 1937, Serial No. 173,561

3 Claims.

"ihis invention relates to a means of biasing therinionic tubes in circuits such as oscillators and arnplihers. Generally, it utilizes the variable resistance characteristics of Thyrite 'i'hyrite is a material which was developed by the General Electric Company ior lightning arresters and has the distinctive characteristic that the current rises very rapidly with very small increases in lvoltage impressed across it. llihen the voltage is doubled the current goes to something like ld times the initial value.

liteierences to this material will be found in the General Electric Review for Eebruary,1930; June,

i930; April, 193i; and May, 1934; and in the i Transactions ci the American institute oi Electrical Engineers for April, i930.

in the past bias voltage has been obtained trom separate voltage supplies such as rectiiiers, motor generators or batteries. These are eirpensive and introduce additional possibilities oi failure.

Grid leak bias has been used in the past When the tube in question draws grid current. In the case oi an oscillator or amplifier this type of bias may endanger a tube since if for any reason the oscillator stops oscillating or the drive to the ampliiier fails, the bias is removed and the tube will draw successive plate current.

Eias is sometimes provided by causing the anode current oi the tube to flow through a resistance connected between the cathode and the negative of the anode current supply source, the grid circuit being connected to the negative of this supply source. This system of bias is very. often unworkable in connection with tubes working class C when used in services such as telegraph. In this case when the telegraph key is up and no excitation is furnished to the ampliiier a relatively high value of anode current must iiow to produce the bias and the power represented by this current is all dissipated in the tube. If a higher resistance is used then the bias is excessive when the key isv of my invention. Y

In the drawings:

Fig. 1 shows a transmitter.

Fig. 2 shows a Thyrite bank.

Fig. 3 is an explanatory graph.

The accompanying drawings show schematically the circuits of a class C telegraph transmitter in which my invention has been incorporated. l .and 2 are tubes` in an amplifier stage, the plate current being supplied through switch 3 and returning through ammeters l yand 5, lead i 6 and Thyrite resistor bank 1 to groundand to the negative side of the anode supply. Excite@I tion is supplied from earlier stages marked 8 to the grids of l and 2. The excitation is keyed in p any suitable way through lead' I" in the earlier For the marking condition excitation causes grid current to flowthrough result in a negative i and t.

E01 the spacing condition no excitation is supplied and, accordingly, no grid current dov/s and thereiore no bias is supplied from this source. This tends to allow a current to how through the anodes oi tubes i and it and through the Thyrite resistor l. e voltage drop across "i causes the cathode to be held at a positive voltage with respect to grod. Since the grids are maintained at ground potential by virtue oi being connected to ground through grid leaks t and it their potential is negative with respect to that of the cathode. it. condition oi equilibrium is reached in which the bias voltage built up across "i allows just the required' amount oi anode current to flow through tubes i and t to produce this bias voltage.

Under the conditions assumed in the drawing, which is illustrative oi a trsmitter which I have constructed, this equilibrium occurs when grid leaks 9 and l 0, which grid bias on the grids oi tubes the voltage across l is 129 volts and when the' current flowing through 'l is 34 milliamperes. At

the voltage assumed this represents an anode dissipation oi 102 watts in each tube. For the marking condition the excitation allows more anode current to flow. This is assumed to equal one ampere total.

Now if Thyrite- 'l hada constant resistance this one ampere would build up a voltage across it of 3800 volts. The peculiar properties are such that as the voltage across it is increased the resistance drops in accordance with a definite law. By virtue of this drop in resistance one ampere actually builds up a voltage of only 309 volts across 1. 'Ihls voltage plus the bias obtained by the grid leak action gives a total bias of the value for best operation.

The characteristics of the Thyrite 1 and the value oi' resistors 9 and I 0 can be adjusted as required.

One modification of my invention is shown in the stage immediately preceding the stage I 2. In this stage marked 8, 2| and 22 are tubes in an ampliiler, the plate current being supplied through switch 23, and return through ammeters 24 and'25, lead 26 and "'I'hyrite resistor bank 21 to ground and to the negative side of the plate supply. Excitation is supplied from earlierstages 28 to the grids of 2| and 22. This excitation is keyed in stage 28. For the'marking condition excitation ycauses grid current to flow through grid leaks 29' and 3l. 1This current results in a.

wave from coming through stage l tubes '2| and .g

22 must be biased beyond cutoff. Cathode return bias and grid leak bias alone cannot maintain these grids at a voltage beyond cutoff. To give this bias voltage, current from the 3000 volt source 3i is supplied to 21 through resistors 32 and 33. For the spacing condition 20 milliamperes f flows through 21, building up a voltage of 212 volts.

In order for this to be true the `resistance of 21 must be 10,600 ohms. If this were ordinary resistance then for the marking condition, when the anode current flows through this resistance the voltage would be excessive. When Thyrite is used the voltage only rises from 212 volts for the spacing condition to 376 volts for the marking condition.

A tap is provided on Thyrite bank 21 which supplies bias for tube il by making its cathode 106 volts positive with respect to its grids for the spacing condition. This protects this tube from excessive anode current which it might otherwise draw if its excitation failed.

This invention will be of value in connection with many types of electric power equipment, including rectiers for radio transmitters, power distribution systems, etc.

Reference is made to Usselman application Serial No. 103,643, filed October 2, 1936, now Patent #2,201,211, granted May 21, 1940, as illustrative of a manner in which the Thyrite banks 1 and 21 may be constructed.

The stage 8--and the stage formed of the tubes l and 2 may be of the frequency multiplier type in which event the plate circuits are tuned to successively higher odd harmonics of the excitation frequency supplied from the output of tube M which, also, if desired, may be made to operate as a frequency multiplier. In this event the plate circuit of tube Rl is tuned to some desired harmonies of the radio frequency excited so marked on the drawings.

The Thyrite bank 200 and tap 202 are used to supply screen grid voltage for `the tubes of the transmitter. Although the Thyrite bank 200 is in series with resistors 312 and 204 in some cases it will be found desirable to connect this bank directly across a rectifier source. Taps to the Thyrite bank may then be used to supply voltages of the same or diierent values to the various .screen grids, plates, etc., in which'case the Thyrite bank 200 will act as a Thyrite potentiometer. Such a Thyrite potentiometer, when connected across the output terminals of a rectier, may be used to supply either positive bias as already described or, the positive terminal may be grounded, in which case the taps to the potentiometer may be used to supply negative bias to various grids of the tubes.

I have found that a Thyrite bank when used as a potentiometer across a rectifier improves the regulation of the rectier. As heretofore used, ordinary resistors across a rectiiier require a greater waste of power than that required by the Thyrite to obtain the same degree of voltage regulation. A further advantage in using a.

Thyrite potentiometer across a rectifier resides in the fact that greater smoothing action is obtained. This will be exlilanedin greater detail by referring to Fig. 2. i

The Thyri bank, as described in the Usselman patent referred to hereinabove, is connected in series with the smoothing reactor 30| and rectier 302. The smoothing condenser 304 is connected in shunt to the "Thyrite bank. Either terminal 306 or 308 may be grounded. Over any one of the fins taps t may be taken to supply positive bias in the event that terminal 308 is grounded or' a negative bias in the event that terminal 306 is grounded. The choke or filter reactor 300 serves to maintain a constant current flow from the rectiiier.

This arrangement will be found to give much better regulation or, rather, a much better regulated voltage at the taps t as compared to a system in which an ordinary resistance potentiometer is used.

Fig. 3 is an explanatory graph on a logarithmic scale showing the change in current through the Thyrite bank with various changes in voltages impressed.

I claim:

1. In a telegraph system, an amplifier, including a thermionic discharge tube having an anode, a cathode and a grid and having grid-cathode and anode-cathode circuits connected thereto, said amplier being characterizedl in that grid current ows in said grid-cathode "circuit during marking intervals in normal operation, means for biasing said tube during marking intervals, including a grid leak and condenser combination in said grid-cathode circuit, and additional means for biasing said tube during spacing intervals including a resistor in a common portion of said grid-cathode and anode-cathode circuits, the resistance of said resistor varying inversely as the current flow therethrough whereby the bias produced by said resistor is not substantially increased during marking intervals.

2. In a telegraphy system, an electron discharge device having electrodes including an anode a cathode and a control grid, alternating current circuits connected to said control grid and cathode and said anode and cathode, means for causing altern-ating current the amplitude of which increases and decreases in accordance with marking and spacing respectively to flow in said circuits, a grid leak and condenser combination having a terminal connected to said control grid, a resistance the potential drop -through which increases at a rate less .than linearly with increase of current flow therethrough connected between said anode and cathode, a connection from another terminal of said grid leak and condenser combination to a point on said non-linear resistance, the potential drop in said grid leak due to grid current flow .therethrough supplemented by the potential drop in said non-linear resistance biasing said device for class C operation in the presence of alternating current of amplitude characteristic of marking in said alternating current input circuit, the potential drop in said nonlinear resistance, in the presence of alternating current characteristic of spacing in said alternating current input circuit serving to compensate or the decrease of potential drop in said grid leak resistance.

3. A system as recited in claim 2 wherein a source of potential is connected in shunt to said non-linear resistance to increase the flow of current therethrough to bias said tube to cut-01T in theipresence of alternating current in said input circuit characteristic of spacing.

JALES LESLIE FINCH. 

